A new generation of very high frequency (VHF) radios must include a mode of operation which has D8PSK (differential 8 phase shift keying) modulation, in accordance with International Commercial Aviation Organization (ICAO) Mode 2 waveform specification. The received radio signal may be corrupted by carrier frequency offset (Doppler) of up to + or xe2x88x921800 Hz due to relative path motion plus relative transmit-receive frequency uncertainty.
A procedure is required that will robustly detect an incoming message, estimate the Doppler present and set the symbol timings so that the message can be demodulated accurately. This invention addresses these problems and provides a solution.
The preferred embodiment is useful in a radio receiver adapted to receive a carrier signal having a Doppler shifted frequency. The carrier signal carries a coded signal having variable amplitude and phase which represents symbols coded by phase. The symbols are divided into a preamble portion having a predetermined code and a message portion. In such an environment, the preferred embodiment processes the coded signal by storing samples of the coded signal representing at least some of the predetermined symbols of the preamble. The stored samples have variable amplitudes and are divided into a plurality of sample groups. One sample group of the various sample groups is identified as having a predetermined amplitude relationship with respect to other of the sample groups. The samples of the coded signal are selected in response to the identification of the one sample group. The Doppler frequency of the selected samples is estimated, and the message portion of the coded signal is delayed while the identifying, selecting and estimating proceeds. The Doppler frequency of the coded signal is reduced in response to the estimated Doppler frequency in order to generate a Doppler reduced coded signal. The Doppler reduced coded signal is filtered in a filter matched only to the transmitted message, but including carrier offset, and the message portion is demodulated in response to the Doppler reduced coded signal.
The storing of the samples preferably is carried out by digital memory and the remaining portions of the above-described technique are preferably carried out by a digital processor.
By using the foregoing technique, an incoming message may be detected in a noisy background and any Doppler frequency present may be efficiently estimated. The technique is robust in time varying channels and is very efficient and conservative of processor time.